Surveillance cameras, that is video cameras associated with recording equipment for security purposes, are widely found in and around commercial and residential buildings. These cameras are considered extremely useful in the deterrence of crime and vandalism as well as the capture and prosecution of culprits. Surveillance cameras associated with motor vehicles of all types are much less prevalent, and are primarily so-called dash-cams, which record one or two scenes (front and rear). Many of these cameras record only when the engine of the motor vehicle is running, but some have a parking mode in which recording is triggered by motion or vibration. This mode is intended to save recording space, since in a vehicle the recording medium is generally an SD flash card with limited capacity, and security is generally a problem during the long periods when the vehicle is idle.
Motion detection is also used with fixed-location surveillance systems to time-mark the points in long hours of video recording where there may be a security interest, such as a possible intruder. A “motion detection” function in a surveillance system commonly consists of comparing successive frames of a video stream and identifying differences. Differences in a scene from one frame to another that exceed a preset threshold are declared to be “motion.” This is a simplistic approach, but suitable to the limited processing power available. Since the approach is susceptible to scene changes that are of no security interest (for example, pedestrians on the other side of a street or tree limbs moving with the wind), such systems also provide a means for the user to manually “mask out” parts of the scene to avoid false positives. The user may be provided with a graphical user interface displaying the scene from a camera overlaid with a grid. The user then selects squares of the grid to be ignored, or masked, for the purpose of motion detection. The scene itself is usually recorded in its entirety. This method is reasonably effective when surveilling a fixed location and where it is possible to angle the camera to allow certain parts of the scene to be isolated and masked to exclude false positives. For example, if a camera can be mounted on an exterior wall and angled downward in such as way that a driveway can be separated in its view from the alley it connects to, then the alley can be masked, and motion detection limited to changes that occur in the driveway. However, this method is not suitable for surveillance from a vehicle, since the vehicle can in principle be parked anywhere and it is not convenient for the vehicle operator to manually review and mask the scene from each camera every time the vehicle is parked. Furthermore, it is not suitable where a camera view must be substantially level with the ground, such that background scenery registers in the same image region of the image as a potential security interest, for example, a person moving in an area where there is a busy street in the background.
Another well-known method of motion detection is the Passive Infra-Red (PIR) detector, which relies on infrared radiation emitted from warm objects (such as humans) that can be distinguished from background infrared. A PIR detector is commonly used to control outdoor lights. However, it will not reliably detect people ‘bundled up’ with layers of clothing, or vehicles that are at the same temperature as the surroundings. Also, since its range of detection is controlled by its gross sensitivity and the level of infrared emitted, it is not possible to accurately define a range of proximity that it can be limited to. Therefore what is needed is a better method of detecting and discriminating motion that may be of interest from a security standpoint from motion of no interest in a surveillance system.